memtable: delete merge value for range deleteion
memtable: delete merge value for range deleteion

Summary: Closes https://github.com/facebook/rocksdb/pull/1797

Differential Revision: D4448004

Pulled By: ajkr

fbshipit-source-id: 3ffc27c

vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,wat-ze-hex/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,wat-ze-hex/rocksdb,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb